Conventionally, a TN (Twisted Nematic) liquid crystal display device is generally used as a liquid crystal display device, however, in order to improve moving image visibility, an OCB liquid crystal display device characterized by high speed response is proposed.
As shown in FIG. 1, in the OCB (Optically Compensated Bend) liquid crystal display device 100, an OCB liquid crystal 106 is sandwiched between an array substrate 102 and a counter substrate 104. In the array substrate 102, an array electrode 110 is formed on an upper surface of an insulating glass substrate 108, and a phase difference plate 112 and a polarizing plate 114 are bonded to a lower surface of the glass substrate 108. On the other hand, in the counter substrate 104, a phase difference plate 116 and a polarizing plate 118 are bonded to an upper surface of a glass substrate 120, and a counter electrode 122 is formed on a lower surface of the glass substrate 120.
In this liquid crystal display device 100, in a state before power is turned on, as shown in FIG. 2A, the orientation state of the liquid crystal 106 is in a spray orientation state. Then, power is turned on to apply a relatively large voltage to the liquid crystal 106 in a short time by voltage application means, and as shown in FIG. 2B, the orientation of the liquid crystal 106 is caused to transition to a bend orientation state. The feature of the OCB type is that an image is displayed by using this bend orientation state.
In this OCB liquid crystal display device 100, in order to keep the bend orientation state, a reverse transition prevention voltage is applied in each frame for a period with a specific ratio or more, and the reverse transition to the spray orientation state is prevented. At this time, when the reverse transition prevention voltage is made equal to the optimum black display voltage, a high contrast is ensured, and moving image visibility can be improved.
In order to prevent the occurrence of the reverse transition from the bend orientation to the spray orientation, it is necessary to apply the reverse transition prevention voltage of a specific voltage or higher for a specific period or longer.
Accordingly, in the case where it is necessary to set the reverse transition prevention voltage to be higher than the optimum black display voltage, there is a problem that black display quality must be sacrificed.
Besides, in order to keep the black display quality, it is necessary to make the reverse transition prevention voltage substantially equal to the optimum black display voltage. However, in this case, it is necessary to prevent the reverse transition by setting an application period of the reverse transition prevention voltage to be long, that is, by setting the period (black insertion ratio) in which the reverse transition prevention voltage is applied in a frame to be long (high), or by setting a white display voltage to be high. However, in such a case, since the display time ratio (display time occupied in the frame period) is reduced, or the white brightness is reduced, there is a problem that the use efficiency of light is remarkably impaired.
In order to solve this problem, JP-A-2003-279931 proposes to change a black insertion ratio and a black insertion voltage (reverse transition prevention voltage) according to the peak brightness of a video signal.
However, in the method of JP-A-2003-279931, there remains a problem that the black display quality in a video signal in which white and black are mixed in a selected line is sacrificed.
Then, in view of the above problems, the invention provides an OCB liquid crystal display device in which the black display quality is not sacrificed even in a video signal in which black and white are mixed in a selected line, and a driving method of the same.